This application pertains to the art of air filtration. The invention is particularly applicable to filtration systems for removing particles from 0.1 microns or less to over 100 microns from air which is heated to a temperature between ambient air and 1500.degree. F. or higher. The invention is further applicable to systems which operate at relatively high air to filter area ratios on the order of 30 to 100 CFM per square foot of filter media. Air filtration systems operating with these parameters find ready application in conjunction with foundry operations, coal-fired boilers, incinerators, and other high temperature processes. It is to be appreciated, however, that the invention is also applicable to filtering particles of other sizes, air at other flow rates and other temperatures, and effluent from other sources.
Heretofore, various types of filtration systems have been used for filtering air. The prior art includes cloth filter or bag house systems, electrostatic precipitators, wet scrubbers, and mechanical collectors such as cyclone, gravitational settling, or impingement separators. The cloth filter systems have had several drawbacks. Commonly, the air flow to filter ratiio is limited to about 2 to 10 CFM per square foot of effective filter area. The air must be precooled before filtering to a temperature of less than 550.degree. F. to prevent the heat from destroying the filter bag cloth. The filter a relatively large volume of air, large plant areas must be dedicated to relatively expensive, large volume filter systems. The mechanical collectors are ineffective for removing submicron particles from the effluent gases. Electrostatic air cleaners are relatively expensive to install and have large space requirements. Further, electrostatic precipitators operate more efficiently when the effluent flow is substantially constant and when there is a controlled amount of sulfur oxides in the effluent stream. The presence of the sulfur oxides may result in conflict with air pollution regulations. The wet scrubbers are commonly ineffective for removing particles smaller than 0.5 microns. Further, wet scrubbers are relatively expensive to operate because a relatively large amount of power is required to create the large pressure drop necessary to remove the smaller particles and because water treatment facilities are required for recirculating the treatment water. In the winter, when shut down, the water is apt to freeze and when operating discharged water vapor tends to form a highly visible steam plume.
Another prior art air filtration system is described in U.S. Pat. No. 3,948,623 issued Apr. 6, 1976. In this patent, the effluent to be cleaned is drawn through a cyclonic agglomerator and into a plurality of filter modules by an exhaust blower. Each filter module has a plurality of vertical cylindrical screen units with a rotatable compressed air or water spray cleaning mechanism inside each one. Each filter module is divided into compartments such that one compartment can provide filtering while the other is removed from the effluent flow for cleaning. A cleaning cycle is initiated when the flow rate of effluent through one of the filter modules drops below a predetermined minimum. In addition to the rotating water spray cleaning mechanism, a propane gas explosion sonic shock wave generator is also disclosed. However, in high temperature applications, propane shock wave generators tend to be ignited prematurely by hot portions of the filter assembly. Further, propane gas shock wave generators and rotating compressed air or water spray cleaners are not readily adjustable to control accurately an amount of filter cake left on the filter media. Another problem with this filtration system is the difficulty in gaining access to the filter assemblies for repair and replacement. Yet another drawback is that heat energy in the effluent gas is lost.
The present invention contemplates a new and improved filtration system which overcomes the above-referenced problems and others. The present invention provides an air filtration system which removes submicron particles from large volumes of gas at elevated temperatures.